The invention relates to a controller of a hybrid vehicle, which can execute fuel cut during vehicle deceleration for as long as possible and which smoothly switches the engine to an idle-driving state.
A vehicle is equipped with a fuel-cut controller in which fuel supply to an engine is interrupted when a condition for fuel cut execution is met during deceleration of the vehicle, and in which the fuel supply to the engine is returned by lifting the fuel cut when the engine speed drops to a predetermined return engine speed, reducing the unburned gas and fuel consumption during deceleration.
Some motor vehicles are of a type commonly called a hybrid vehicle having an internal combustion engine driven by combustion of fuel and an electric motor (hereinafter referred to as xe2x80x9cmotorxe2x80x9d) driven by electric energy, the motor being directly connected to the engine and also having a power-generating function. The hybrid vehicle is equipped with: an engine; a motor directly connected to an output shaft of the engine; an engine controller for controlling the operating state of the engine; and an assistive motor controller to control operating states of the motor. The engine and motor controllers detect the operating states of the engine and the motor respectively, which are controlled in association with one another. Such a control system attains a high level of required performance (such as fuel efficiency, lower values of detrimental components in exhaust gases, and power performance). The motor controller is connected to the battery which supplies the motor with electric power and is charged by electric power from the motor.
Such a controller of a hybrid vehicle is disclosed in Japanese Patent Laid-Open No. 10-23604 which controls the motor to be driven to assist the engine for prevention of engine stall when engine speed is equal to or less than a predetermined value and the amount of change in engine speed is decreasing during vehicle driving.
However, in a conventional fuel cut control, as shown in FIG. 7, fuel supply to the engine is interrupted (cut) by starting a fuel cut control when a throttle opening is small and an idle switch is turned on in the decelerating state (see time X1). Then the fuel supply is cut for a predetermined period (M1) until engine speed drops, with engine remaining braking, to an engine speed for return (see time X2). Then the fuel supply is restarted by stopping the fuel cut control when the engine speed drops to the return engine speed. This return engine speed is usually set at a value ranging from 1300 to 1500 rpm. After engine speed drops to the return engine speed shown at time X2, the vehicle is decelerating while consuming fuel. Then after a predetermined time (M2) when the clutch is disengaged or the transmission is in a neutral position shown at time X3, engine speed converges at an idle engine speed.
On the other hand, as shown in FIG. 8, fuel supply to the engine is prevented (cut) by executing a fuel cut control when the throttle opening is less and an idle switch is turned on in the decelerating state (see time Y1). Also, when the engine is in a free state by clutch disengagement (see time Y2) after a predetermined time period (M3) during engine braking, then the engine speed drops to less than the return engine speed, so that the fuel supply is restarted by stopping the fuel cut control. Then after a predetermined time period (M4), when engine speed is gradually decreased (time Y3), engine speed converges at an idle engine speed.
However, the return engine speed needs to be set at a certain value such that fuel supply restarts at an early stage after fuel cut is executed, which otherwise brings engine stall even if fuel supply is restarted. On this account, the return engine speed needs to be set at a comparatively high speed having rather much margin (value), resulting in less fuel consumption improvement.
In addition, the above prevention arrangement intends to assist the engine before engine stall for some reason, which thus cannot continue fuel cut during vehicle deceleration for as long as possible.
In order to obviate or at least minimize the above use of fuel during vehicle deceleration, the present invention provides a control system for a hybrid vehicle having an internal combustion engine, a motor, and a battery. The motor is connected to the output shaft of the engine and has driving and power generating functions. The battery is connected to the motor. The control system includes: a deceleration detector to detect deceleration of the vehicle, a fuel cut control device for executing fuel cut control, means for stopping fuel supply to the engine when a fuel cut executing condition is met while the vehicle is decelerating, and restarting the fuel supply to the engine by canceling the fuel cut control when engine speed drops to a predetermined return engine speed. An assist controller drives the motor to execute assist control when engine speed drops to less than the return engine speed.
In the fuel cut control device during vehicle deceleration of the present invention, at engine side, a return engine speed is set to a speed in which the engine inevitably stalls, which is the engine speed set as low as possible, so that fuel cut is executed as long as possible to improve fuel consumption. Also at motor side, assist-control is executed such that engine speed is maintained so that engine stall does not occur, so that the engine speed converges at an idle speed smoothly without any detectable uncomfortable feeling.